This invention relates to shape selective crystalline silicate zeolite catalysts and their use in hydrocarbon conversion processes. More particularly, this invention relates to such zeolites containing an intermetallic component, e.g. HZSM-5 containing platinum silicide, and to the use of such catalysts for a variety of hydrocarbon conversions, e.g. isomerization of waxy paraffins.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline silicates having a definite crystalline structure as determined by X-ray diffraction within which there are a number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for absorption molecules of certain dimensions while rejecting those of larger dimensions, these materials are said to be "shape selective" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum plus silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,242); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite beta (U.S. Pat. No. 3,308,069); zeolite XK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); ZSM-35 (U.S. Pat. No. 3,016,245); ZSM-38 (U.S. Pat. No. 4,046,859) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
These and other zeolites have been widely investigated as catalysts for a variety of hydrocarbon conversion processes including isomerization of waxy paraffins. Thus, for example, U.S. Pat. No. 4,419,220 describes a process for catalytically dewaxing a variety of feedstocks containing waxy normal and slightly branched paraffins over zeolite beta containing a hydrogenation component such as platinum or palladium. During the processing, the n-paraffins become isomerized to iso-paraffins and the slightly branched paraffins undergo isomerization to more highly branched aliphatics resulting in product of reduced pour point.
The modification of zeolites X and Y optionally containing a hydrogenating metal component with an organosilane capable of entering the pores, or channels, of these zeolites is known from U.S. Pat. No. 4,451,572. The organosilane modified zeolite either before or after the deposition of the optional metal component is heated, usually at above 300.degree. C., in an inert or reducing atmosphere to form a stable surface resulting from condensation/polymerization of the silated surface. The amount of organosilane employed is such as to result in significant constriction of the pore diameters and a consequent reduction in pore volume. The modified zeolite is said to be useful in hydrodewaxing, selective paraffin cracking paraxylene isomerization and the preferential acid catalyzed reactions of linear or slightly branched paraffins or olefins.
U.S. Pat. No. 4,507,401 describes supported intermetallic compounds produced by a two-step process. In the first step, a supported metal is formed, for example, by solvating a metallic salt such as nickel nitrate and applying it to a support medium. The treated support body is then heated in the presence of a reducing agent to produce elemental metal. The supported metal is then treated with a reactive organometallic or metal hydride compound to yield a supported intermetallic compound. The catalysts are said to be useful for the preferential catalysis of dehydrogenation rather than hydrogenation processes.